Echo signals reflected by things in water such as planktons and small dust and the like and by the seabed and resulting from ultrasonic signals emitted from a transmitting-and-receiving transducers mounted on the bottom of a ship have Doppler shifted frequencies due to the Doppler effect and allow the detection of the speed of the own ship or the velocity of a water current by measuring the Doppler shifted frequencies.
For example, when the speed V of the ship is desired to be detected, an ultrasonic signal (having a carrier frequency "f.sub.o ") is emitted downwardly in an oblique direction (at an angle .theta. with respect to the horizontal) by means of a transmitting-and-receiving transducer as shown in FIG. 4. A Doppler frequency shift "f.sub.d " included in echo signals reflected by the seabed will be as follows: EQU fd=2V.multidot.fo.multidot.cos .theta./c (1)
where "c" is the speed of sound in the water. Accordingly, the velocity V of a ship can be obtained from the following equation which is a modification of the equation (1). EQU V=fd.multidot.c/(2fo.multidot.cos .theta.) (1')
Thus, the Doppler frequency shift "fd" needs to be detected and the following three methods are conceivable to obtain it.